If publication payments are enabled, the following appears on a submission's Editing page, under Scheduling, if the author has not yet paid the publication fee:
"Publication Fee not paid. To schedule item for publication notify author to pay fee or waive fee."
However there is no email reminder/notification option available there. There should be an email link that goes directly to a prepared email saying something to the effect that the author has to pay the fee via {url} or otherwise provide further reasons for requesting the fee be waived.